Ultrasonic imaging apparatus for reading and decoding machine-readable matrix symbols

ABSTRACT

There is provided an apparatus and method for reading matrix codes comprising an ultrasound transducer  20  which comprises transmitter and receiver electrodes, a receiver unit  24  connected to the receiver electrodes for processing the signals received by the receiver electrodes, and a transmitter module  21  connected to the transmitter electrodes, the transmitter module transferring an electronic signal to the transducer, where the signal shape is selected between a number of signal shapes according to the characteristics of the matrix code to be read.

This application is a Continuation of U.S. Ser. No. 13/124,200, filed 8Jul. 2011, which is a National Stage Application of PCT/NO2009/000360,filed 14 Oct. 2009, which claims benefit of Serial No. 08166549.9, filed14 Oct. 2008 in the European Patent Office and which applications areincorporated herein by reference. To the extent appropriate, a claim ofpriority is made to each of the above disclosed applications.

BACKGROUND OF THE INVENTION

Machine readable matrix symbols are a special case of opticallyscannable symbols which often use circles or square based cells insteadof variable widths of spaces and lines, such are used in binary barcodes, to represent the data.

Historically, the automatic identification industry has relied onoptical cameras for reading matrix symbols. Optical cameras possess goodperformance when reading matrix symbols printed on papers or labels, butmay have problems when reading matrix symbols that are directly markedon substrates using marking methods such as dot-peen or laser Moreover,when direct marked matrix symbols are painted, the challenge of readingthe symbols using optical contrast readers becomes even larger.

Ultrasound as a means of reading matrix symbols is possible if thatbackscattered ultrasound signal corresponding to the matrix symboldiffers from that corresponding to the substrate where the matrix symbolis comprised. This is achieved if the matrix symbol is applied to thesubstrate using materials with different acoustic impedance propertiesthan the acoustic impedance properties of the substrate, or if theultrasound propagation time to the matrix symbol is different than tothe substrate.

Using ultrasound as a means for reading matrix symbols has beensuggested in U.S. Pat. No. 5,733,811. They proposed an apparatus forreading matrix symbols using an ultrasound transducer which isphysically scanned by moving it over the component bearing thetwo-dimensional symbol in a raster pattern.

SUMMARY OF THE INVENTION

The object of the invention is to provide an improved apparatus andmethod for reading matrix codes/symbols.

The apparatus according to the invention reads the matrix symbolmarkings using a transducer comprising a plurality of elements thatforms the image points/pixels. The matrix symbol is imaged by applyingthe transducer steady/stationary over the substrate surface. Hence, theprinciple of reading the matrix symbol is different from U.S. Pat. No.5,733,811 since no physical movement of the transducer is necessary.There are several benefits of using such imaging method contrary tousing a transducer element that physically moves over the substratecontaining the matrix symbol. First, the imaging method is potentiallymuch faster since parallelisation is easily implemented by using severaltransducer elements simultaneously. Second, since there are nomechanical movements involved the imaging is more robust and perfectalignment of the image pixels are always satisfied. Hence, we reduce anypotentially geometrical distortions of the imaged matrix symbols. Third,since the transducer contains no physically moving parts it renderpossible easy use of dry coupling pads. Fourth, since the design of theapparatus uses few connectors to the transducer compared to the numberof transducer elements and off the shelf components, the production costis low.

The object of the invention is to provide an apparatus for readingmatrix symbols that enhances features that discriminate the pattern inthe matrix symbol.

The apparatus according to the invention further can construct a videostream using features extracted from the received ultrasound signals. Byexploiting the transmission scheme and the underlying features, highcontrast ultrasound images of the matrix symbol may be constructed. Thiswill provide the potential for reading and decoding a larger selectionof matrix symbols.

The object of the invention is achieved by means of the features of thepatent claims.

An apparatus according to the invention for reading matrix codescomprises in one embodiment an ultrasound transducer which comprisestransmitter and receiver electrodes, a receiver unit connected to thereceiver electrodes for processing the signals received by the receiverelectrodes, and a transmitter module connected to the transmitterelectrodes, the transmitter module transferring an electronic signal tothe transducer, where the signal shape is selected between a number ofsignal shapes according to the characteristics of the matrix code to beread.

In one embodiment the apparatus further comprises a pulse evaluationunit connected to a memory device, the pulse evaluation unit evaluatinga number of pulse shapes against N criterions to save the best pulseshape for each criterion in the memory device.

The apparatus may comprise a pulse selection module connected to thetransmitter module for selecting one of N pulse shapes for supplying tothe transmitter module. The N pulse shapes may be stored in a memorydevice.

A multiplexer may select one of the N pulse shapes from the memorydevice.

In one embodiment, there is provided a strategy selector for choosing astrategy for selecting the pulse shape from the memory device.

The strategy may be to select pulse shape automatically or by a user,according to part type.

In one embodiment, the apparatus according to the invention comprises animage construction module connected to the receiver unit, and the pulseselection module is arranged to select signal shapes which providesenhanced images produced by the image construction module.

The pulse selection module may be arranged to select the pulses whichprovides images with best contrast.

The apparatus according to the invention may also comprise a featuredatabase which comprises a number of image features provided from imagesgenerated with a number of different pulse shapes. Information on thepulse shapes associated with the image features may also be stored inthe database.

In one embodiment, the apparatus according to the invention comprises anextract features module and a compare module. The extract featuresmodule extracts image features from the images produced by the imageconstruction module and the compare module compares the featuresextracted by the extract features module to the features of featuredatabase and provides the pulse shape associated with the feature of thefeature database having the best match to the extracted feature.

In one embodiment, the transmitter and receiver electrodes form amatrix.

The pulse selection module may be incorporated in the transmittermodule.

The pulse selection module may be connected to the receiver unit fortransmitting information on the transmitted pulse.

The receiver unit comprises in one embodiment a receiver module whichcomprises at least one multiplexer, an amplifier, and an analogue todigital converter, a receiver control module controlling themultiplexer(s) and a signal processing module.

The signal processing module may in one embodiment be arranged toprocess the signals from the receiver module and comprise time gatingmeans, averaging means and feature extraction means.

The signal processing module is in one embodiment connected to the pulseselection module in order to use information regarding the signal shapesin the signal processing.

In one embodiment of the apparatus according to the invention, itcomprises an image enhancement unit.

The invention may also comprise solutions for connecting a substratewith matrix symbols to various settings of the apparatus. We herebyrefer to the “part” as a particular substrate with a matrix symbolmarked on it. Since the ultrasound image varies substantially from onesubstrate and marking method to another, it is important to organize theuseful settings of the apparatus. The “part” is thus included in theuser interface. A list of parts is stored in the apparatus, for examplein the receiver unit. Along with a given part, information about thesettings (such as pulse pattern, time gate information, and featureextraction selection) is stored. Hence, when the user selects a givenpart from the list of parts, the system automatically sets the settingsof the apparatus that are stored along with the part.

The user has also the possibility to create its own list of parts byeither manually entering the information or by a training functionalitythat automatically alters the settings of the apparatus and selects theoptimal setting for the part.

The information related to a successful decoded matrix symbol isavailable for the user by means of a configurable data output interface.

The invention will now be described in more detail by means of examplesas shown in the enclosed figures.

FIG. 1 illustrates the principle of the ultrasound transducer applied inthe apparatus according to the invention.

FIG. 2 is a block diagram illustrating the different elements of theapparatus according to the invention.

FIG. 3 shows a block diagram of one embodiment of a pulse selectionmodule for use in the apparatus according to the invention.

FIG. 4 shows a block diagram of one embodiment of an automatic pulseshape selection.

FIG. 5 shows a block diagram of a pulse shape construction algorithm.

FIG. 6 a-d show example embodiments of transmitter and receiver module.

FIG. 7 a-c show different embodiments of the signal processing module.

FIG. 8 shows examples of images produced by the apparatus according tothe invention.

FIG. 9 shows an embodiment of an image enhancement process.

FIG. 1 illustrates the principle of the apparatus 10 according to theinvention. An ultrasound transducer comprises transducer elements 11.The transducer elements transmit and receive ultrasound wavestowards/from an object 14 which comprises a matrix symbol 13. The matrixsymbol may be covered or uncovered. The cover may for example be a layerof paint. The area covered by the matrix symbol is imaged by theultrasound transducer by altering the transducer elements 11 such thatthe area covered by substrate hosting the matrix symbol is imaged in aunique manner. The matrix symbol is imaged by applying the transducerstationary over the matrix symbol. In the figure it is used a drycoupling medium which acts as the sound transmission medium forultrasound energy transmitted and received by the transducer. Thecoupling medium is needed in order to ensure coupling of the ultrasonicenergy generated by the transducer into the substrate where the matrixsymbol is comprised, and to provide a distance between the transducerand the matrix code in order to delay the backscattered echo such thatit is not received simultaneously as the transducer is transmitting. Thecoupling medium may be a dry coupling medium or a liquid couplingmedium. The apparatus may be used under water, and in that case thewater may itself constitute the coupling medium.

A dry coupling is more feasible and easy to use in an industrialenvironment than a liquid coupling medium. However, the dry couplingneeds to be flexible and soft enough such that it adapts to thesubstrate surface where the matrix symbol is comprised and it need toprovide low damping of the ultrasonic signal. These properties may forexample be achieved by a dry coupling made of an elastomer.

FIG. 2 is a block diagram illustrating the different elements of theapparatus according to the invention (with one possible embodiment ofthe transducer). The apparatus comprises a transducer 20, a transmittermodule 21, a pulse selection module 22, a transmitter control 23 and areceiver unit 24.

The transducer 20 comprises transmitter and receiver electrodes arrangedin an intersecting pattern, where the intersections form a matrix.

In one embodiment of the invention the transducer 20 is built up of anumber of elongated, parallel driving electrodes arranged on one surfaceof a transducer plate and a plurality of parallel receiving electrodesarranged on the opposite surface of the transducer plate. The drivingelectrodes and the receiving electrodes intersect to effectively form amatrix of individual transducer elements capable of emitting andreceiving ultrasonic beams.

In another embodiment of the invention the transducer 20 is built up ofa number of elongated, parallel combined driving/receiving electrodesarranged on one surface of a transducer plate and a plurality ofparallel grounding electrodes arranged on the opposite surface of thetransducer plate. The driving electrodes and the grounding electrodesintersect to effectively form a matrix of individual transducer elementscapable of emitting and receiving ultrasonic beams. The emission andreception of ultrasonic beams must be done in separate operations byswitching the driving electrodes between a transmit state and a receivestate

In yet another embodiment of the invention the transducer 20 is built upof a number of elongated, parallel driving electrodes on one surface ofa transducer plate, parallel receiving electrodes on the same surface ofthe transducer plate and a grounding plane on the opposite surface ofthe transducer plate. The driving electrodes and receiving electrodesare organized in such a way to effectively form a matrix of individualtransducer elements, e.g. by being orthogonal to each other, capable ofemitting and receiving ultrasonic beams. An arrangement of the crossingdriving and receiving electrodes in a multilayer structure on e.g. aflexible polyimide circuit will provide the crossing electrodearrangement without electrical contact between the driving and receivingelectrodes.

The transmitter electrodes are connected to the output of thetransmitter module 21. This module 21 supplies a transmitter pulsepattern with a particular signal shape corresponding to an input pulseto the transmitter electrode selected by the transmitter control 23. Thereceiver electrodes are connected to the input of the receiver unit 24.The receiver unit 24 comprises in this example a receiver module 25, areceiver control 26 and a signal processing module 27. These modules mayin some embodiments be integrated into one module. In other embodiments,the receiver unit 24 may comprise fewer or additional modules. Thereceiver module 25 senses, amplifies, and samples the signal supplied bya given receiver electrode. Which receiver electrode to handle/processmay for example be selected by the receiver control 26. The output ofthe receiver module is connected to the signal processing module 27which extracts relevant features from the received ultrasound signals.Examples of such relevant features may be time, amplitude, phase,energy, etc. These features are used to construct ultrasound images ofthe imaged surface. The signal processing module may also receiveinformation from the pulse selection module 22, such that the mostappropriate signal processing algorithm is applied, the processingalgorithm being adapted/suited for processing signals originating from aparticular pulse pattern/signal shapes.

The signals from the signal processing module 27 represent data that maybe used to construct an image of the matrix code. The image constructionmay be performed in the apparatus, being either a separate moduleconnected to the signal processing module or integrated in the signalprocessing module. The image construction may comprise imageenhancement, either integrated in the image construction step/module oras a separate step/module. In the embodiment of FIG. 2, imageconstruction and image enhancement takes place in separate steps inmodules 28 and 29 respectively. Image enhancement increases theprobability of a successful decoding. The imaged matrix symbol are thendecoded by the decoding module 30, and displayed by the display module31. The image construction, image enhancement, decoding, and displaymodule may be a part of the apparatus or may optionally be located on anexternal device, e.g. PC.

The decoding module 30 extracts the matrix symbol bit pattern anddecodes it using the appropriate decoding algorithm.

The display module 31 is implemented in order to visualize the imagedsubstrate surface. This will guide the user to provide faster decodingand selection of the most appropriate processing algorithms.

Pulse Selection Module

One embodiment of the pulse selection module 22 is illustrated as ablock diagram in FIG. 3. The pulse selection module may comprise or beconnected to a pulse generator which supplies the transmitter modulewith an electronic pulse pattern that will be converted to ultrasonicpulses by the transducer. Information about how to generate N pulseshapes and the pulse shapes themselves is stored in a memory device 33.One of these pulse shapes are selected by a multiplexer 34. The pulseshape that is selected depends on the strategy chosen by a strategyselector 35 and input to the multiplexer. The selected pulse shape isconverted to an analogue signal in converter unit 36 to drive thetransmitter electrode. The strategy selector is a unit that is eitherset to keep a fixed strategy, to vary the strategy according to inputfrom a user, or to choose strategy based on internal or external input.

The shape of the pulse pattern is constructed to enhance desiredfeatures that will be extracted by the signal processing module and/orthe image enhancement module for the particular part. E.g., if the phaseimage is important to create a high contrast image a pulse shape can beemitted that decreases the probability of selecting a samplecorresponding to the wrong phase. If a matrix symbol that is covered bypaint is to be imaged, a pulse shape can be chosen that correspond to ashort pulse as possible such that the received echoes corresponding tothe various layers can be resolved (with respect to time).

In one embodiment of the invention the strategy is to let the userselect the pulse shape. In another embodiment the strategy is to let theselected pulse shape to be provided by a part file corresponding to agiven matrix symbol. The part files consist of a set of controlparameters optimal for the given matrix symbol, including pulse shapesuitable for imaging and decoding the symbol, features used in imageconstruction, cell size, code size, paint depth, etc. These controlparameters being specific for the type of matrix symbol and substrate(part).

In yet another embodiment of the invention, an automatic pulse shapeselection is implemented, where an image provided by the imageconstruction module 28 of FIG. 2 obtained using a reference pulse shapeand feature extraction (referred to as input image) is evaluated againsta database of matrix symbol images and image features 42. The databaseconsists of matrix symbol images and image features that are obtainedfrom images by using the same reference pulse shape and featureextraction method, and additional information about the pulse shape thatprovided the best image under some criterion.

A block diagram of an embodiment of the automatic pulse shape selection,which may be used in the pulse selection module 22 of FIG. 2 and furtherdescribed in FIG. 3, is shown in FIG. 4. Image features are extractedfrom the input image by the Extract features module 41 and compared inthe Compare module 43 to corresponding image features stored in aFeature database 42. Each set of image features in the feature databasecorrespond to a given pre-analyzed image, and the image features areextracted using mathematical operations on the image. Since the matrixsymbol may be imaged with varying orientation, translation and scale,the features applied need to be orientation, translation and scaleinvariant. The Select best module 44 select the features from thefeature database that represent the best match to the features of theinput image, and the corresponding optimal pulse shape is selected bythe Select pulse shape module 45. Examples of image features to use arespatial intensity features, spatial texture features, edge features, orcombination of features (see e.g. (Takahasi et al, 2000)).

To find the pulse shapes to store in the memory device 33 of FIG. 3,mathematical expressions may be used as evaluation criterions. FIG. 5shows a block diagram of a possible pulse shape construction algorithmwhich may be embodied in a pulse evaluation unit. The input to thealgorithm is a large set of pulse shapes to be evaluated, generatedstructurally (e.g. all possible pulse shapes in a given time intervalsupported by the electronics) or random. The corresponding pulses aresent through the transmitter 51, the medium 52 and through the receiverchain 53. Then, the pulse shapes are evaluated against N criterions C1,C2, . . . , CN, in the Calculate cost module 54 which measures the fitof the pulse shapes against the criterions C1, C2, . . . , CN. Thecalculated performance cost or objective for each pulse shape is thenadded to a list 55, and the best pulse shapes corresponding to eachcriterion are selected by the Select best module 56, and we end up withN optimal pulse shapes corresponding to each criterion. Examples ofcriterions to use are signal-to-noise ratio, main lobe to side loberatio, bandwidth, etc.

Transmitter Module

The transmitter module 21 of FIG. 2 is in a simplest embodiment of theinvention, shown in FIG. 6 a, constructed using a driving amplifier 61connected to the input of the de-multiplexer 62. A transmitter control63 selects the transmitter electrode to be excited by altering theaddress inputs of the de-multiplexer. The transmitter module may beconstructed using of the shelf devices, such as plasma display driversor ink jet printer drivers.

In another embodiment of the transmitter module, shown in FIG. 6 b, thetransmitter electrodes are each controlled independently, by assigning adriving amplifier 64 to each transmitter electrode. The transmittercontrol 65 is now designed to control the gain of the amplifiers. Thisdesign makes it possible to excite several transmitter electrodessimultaneously, which is needed if transmit beamforming is to beperformed.

Receiver Module

The receiver electrodes are connected to a multiplexer in the Receivermodule 25 of FIG. 2, which is controlled by the receiver control means26 (FIG. 2). The output of the multiplexer is connected to a low noiseamplifier (LNA), voltage amplifier, and a low-pass filter. The amplifiedsignal is then sampled by an analog-to-digital converter and the digitalsignal is sent to the signal processing module 27.

In one embodiment, shown in FIG. 6 c, the LNA works as a transimpedanceamplifier and is converting the input current to a voltage.

In another embodiment, the input is connected to a resistor, as shown inFIG. 6 d, and the voltage across the resistor is amplified by the LNA.

Signal Processing Module

For each element in the ultrasound transducer we have N correspondingdigital time series obtained by repeated measurements/operations of theapparatus. Each of these time series is processed by the signalprocessing module. Typical processing operations include time-gating,averaging the N time series, and feature extraction of the averaged timeseries (see FIG. 7 a). The averaged time series are constructed frommultiple recordings from the same electrode, or as the average ofseveral electrodes, or as an average of multiple recordings frommultiple electrodes. The features selected are features thatdiscriminate the differences of the backscattered echo corresponding toa matrix symbol and the substrate. Time gating is necessary in order tocompensate for the delay introduced by the dry coupling pad and possiblypaint layers.

FIG. 7 b shows one embodiment of the signal processing module, where theaveraged time series are possibly rectified and filtered. A sample ofthe filtered signal is selected using some criterion (e.g. the maximumvalue), and the corresponding amplitude, time and phase is the extractedfeatures. These features are particularly useful features to constructan ultrasound image of a matrix symbol. Impedance miss-matches betweenthe matrix symbol and the substrate will be revealed in the amplitude ofthe backscattered echo pulse. The phase is particular useful for imagingdot-peen or laser marked matrix symbols on metal substrates. This isbecause matrix symbol will be constructed with “air!” (air will fill theholes made by dot-peen and laser markings) and the impedance of air willbe much lower than the impedance of the dry coupling pad. Moreover, theimpedance of the substrate will then be much higher than the couplingpad. Hence, one can observe a phase difference of the backscatteredpulse corresponding to the matrix symbol and substrate, respectively.The time feature is efficient when the matrix symbol is covered with,e.g. paint and the amplitude of the backscattered signal is the same forthe matrix symbol and substrate. Optionally a plurality of “selectsample” modules are implemented, each using different criterions toselect samples from the time series. E.g. we may select the maximumvalue, the maximum amplitude value, and the minimum value, and extractthe amplitude, time and phase from all theses sample values.

Another embodiment of the signal processing module is shown in FIG. 7 c,where the analytic signal (Bracewell, 1986) is constructed and used forfurther processing. By rectifying the analytic signal the envelope ofthe backscattered pulse can be obtained. A sample of the envelope signalis selected using some criterion (e.g. the maximum value), and thecorresponding amplitude and time are the extracted features. Also forthis embodiment several samples may be selected from the envelopesignal.

In yet another embodiment of the invention, an automatic signal featureselection is implemented, where an image provided by the Imageconstruction module obtained using a reference pulse shape and feature(referred to as input image) is evaluated against a database of matrixsymbol images and image features. The database consists of matrix symbolimages and image features that are obtained from images using the samereference pulse shape and feature extraction method, and additionalinformation about the feature extraction method that provided the bestimage under some criterion. A block diagram of an embodiment of theautomatic signal feature selection which may be used in the signalprocessing module 27 of FIG. 2 is shown in FIG. 4 and is similar to theautomatic pulse shape selection procedure described above. Imagefeatures are extracted from the input image by the Extract featuresmodule 41 and compared against corresponding features stored in aFeature database 42 in the Compare module 43. Each set of image featuresin the feature database correspond to a given image, and the imagefeatures are extracted using mathematical operations on the image. Sincethe matrix symbol may be imaged with varying orientation, translationand scale, the features applied need to be orientation, translation andscale invariant. The Select best 44 module select the features thatprovide the best match to the features of the input image, and thecorresponding optimal signal feature is selected by the Select featuremodule 46.

Image Construction

Using the extracted features corresponding to the selected transmitterand receiver electrodes, ultrasound images are constructed in the imageconstruction module 28 of FIG. 2. For example, in the embodiment shownin FIG. 7 b, a phase image, time image and an amplitude image isobtained. The constructed images are sent to the image enhancementmodule 29 of FIG. 2, which may be located on the imaging device or onanother device.

The benefit of using the pulse generation with corresponding signalprocessing module as suggested above is illustrated in FIG. 8, where weclearly see the increase of contrast achieved using the proposed pulsegeneration and signal processing algorithms. FIG. 8 a shows an imageconstructed using a pulse generation criterion and corresponding featureextraction that enhances the phase information of the backscatteredsignals. FIG. 8 b shows an image constructed using the energy of thereceived ultrasound signals alone.

Image Enhancement

The image enhancement module 29 enhances the images such that theprobability of a successful decoding is increased. In one embodiment ofthe invention, the image constructed from one feature is processed andenhanced. In another embodiment, image fusion of the images constructedfrom the extracted features is applied in order to construct images ofthe matrix symbol with increased contrast.

In yet another embodiment of the image enhancement module, theultrasound images are constructed like a puzzle (built up by imagefragments) which is useful if one cannot obtain a complete image of thematrix symbol (e.g. it may be marked on a curved surface). An example ofa flow of such a function/process is shown in FIG. 9. Using theconstructed ultrasound images, the Code location estimation 91 estimatethe portion of the image that contain a matrix symbol, and stores thecorresponding pixel indices. The corresponding pixel indices in theimage to display are updated/accumulated whereas the remaining pixelindices (corresponding to locations with no matrix symbol) are leftunchanged.

In yet another embodiment of the image enhancement module, an automaticimage enhancement selection is implemented, where an image provided bythe Image construction module is evaluated against a database of matrixsymbol images/features. The database consists of matrix symbolimages/features that are obtained from previously acquired images, andadditional information about the image enhancement algorithm thatprovided the best image under some criterion. A block diagram of anembodiment of the automatic image enhancement selection which may beused corresponds to the automatic pulse shape selection and theautomatic signal feature selection described above with reference toFIG. 4. Image features are extracted from the input image by the Extractfeatures module 41 and compared against corresponding features stored ina Feature database 42 in the Compare module 43. This is implementedsimilarly as in the automatic pulse selection module. Each set offeatures in the feature database correspond to a given image. Since thematrix symbol may be imaged with varying orientation, translation andscale, the features applied need to be orientation, translation andscale invariant.

The Select best 44 module select the features that provide the bestmatch to the features of the input image, and the corresponding imageenhancement algorithm is selected by the Select image enhancement module47. As in the automatic pulse selection module, examples of imagefeatures to use are spatial intensity features, spatial texturefeatures, edge features, or combination of features.

REFERENCES

-   Bracewell, R; The Fourier Transform and Its Applications, 2nd ed,    McGraw-Hill, 1986.-   N. Takahashi, M. Iwasaki, T. Kunieda, Y. Wakita, and N. Day, “Image    retrieval using spatial intensity features”, Signal Processing:    Image Communications, Vol. 16, pp. 45-67, 2000.

1.-27. (canceled)
 28. An apparatus for imaging an object, the apparatuscomprising: a plurality of transmitter and receiver elements arranged ina matrix, the transmitter and receiver elements being respectivelyconfigured to transmit ultrasound signals towards the object and receiveultrasound signals from the object; a dry coupling configured to act asa transmission medium for the ultrasound signals; and an imageconstruction module configured to construct an image of the object usingfeatures extracted from the received ultrasound signals.
 29. Anapparatus as claimed in claim 28, comprising a signal processing moduleconfigured to: extract the features from the received ultrasoundsignals; and output signals to the image construction module thatrepresent data for constructing the image.
 30. An apparatus as claimedin claim 29, the signal processing module being configured to extractfeatures from the received ultrasound signals that discriminate betweenechoes of the transmitted ultrasound signals, said echoes correspondingto materials in the object having different acoustic impedanceproperties or different ultrasound propagation properties.
 31. Anapparatus as claimed in claim 29, the signal processing module beingconfigured to extract features that include one or more of amplitude,time, energy and phase.
 32. An apparatus as claimed in claim 28,comprising a pulse selection module configured to control a pulse shapetransmitted by the transmitter elements.
 33. An apparatus as claimed inclaim 32, the pulse selection module being configured to control thepulse shape to enhance features extracted by the signal processingmodule.
 34. An apparatus as claimed in claim 32, the pulse selectionmodule being configured to control the pulse shape so that echoes of thetransmitted ultrasound signals corresponding to different layers in theobject can be resolved.
 35. An apparatus as claimed in claim 32, thepulse selection module being configured to control the pulse shape toprovide images with best contrast.
 36. An apparatus as claimed in claim32, the pulse selection module being configured to control the pulseshape in dependence on a set of control parameters.
 37. An apparatus asclaimed in claim 36, the control parameters being specific to the objectbeing imaged.
 38. An apparatus as claimed in claim 36, the controlparameters including a depth in the object.
 39. An apparatus as claimedin claim 29, the signal processing module being configured to select asignal processing algorithm for processing the received ultrasoundsignals in dependence on the pulse shape.
 40. An apparatus as claimed inclaim 28, the image construction module being configured to constructone or more of a phase image, a time image and an amplitude image. 41.An apparatus as claimed in claim 28, each of the receiver elementscorresponding to a pixel in the constructed image.
 42. An apparatus asclaimed in claim 28, the apparatus comprising a transducer, thetransmitter and receiver elements forming the electrodes of thattransducer, and the electrodes being arranged in an intersecting patternto form the matrix.
 43. An apparatus as claimed in claim 28, the signalprocessing module comprising time gating means, averaging means andfeature extraction means.
 44. An apparatus as claimed in claim 28, thedry coupling being capable of adapting to the surface of the object. 45.An apparatus as claimed in claim 28, the dry coupling providing lowdamping of the ultrasound signals.
 46. An apparatus as claimed in claim28, the apparatus being configured to construct an image of the objectbeneath its surface.
 47. A method for imaging an object comprising:transmitting ultrasound signals towards the object and receivingultrasound signals from the object by means of a matrix of transmitterand receiver elements and a dry coupling configured to act as atransmission medium for the ultrasound signals; extracting features fromthe received ultrasound signals; and constructing an image of the objectusing the extracted features.